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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Glycogen phosphorylase

Glycogen phosphorylase releases glucose 1-phosphate from glycogen; it controls glycogen breakdown in liver and muscle.

Updated 2026-11-01

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## Key facts

- Glycogen (n) + Pi → glycogen (n − 1) + glucose 1-phosphate: it cleaves with phosphate (phosphorolysis), not water, saving one ATP.
- It needs pyridoxal phosphate (B6); muscle keeps much of the body's B6 in this enzyme.
- It only cuts α-1,4 bonds and stops 4 residues from a branch.
- Phosphorylated it is active (form a); phosphorylase kinase phosphorylates it.
- Structure 1FA9 (human liver) has glucose in the active site, PLP, and AMP in the allosteric site.

**EC number**
[2.4.1.1](https://www.enzyme-database.org/query.php?ec=2.4.1.1)

**Genes**
PYGL, PYGM, PYGB

**Structure (PDB)**
[1FA9](https://www.rcsb.org/structure/1FA9)

## Regulation

- Liver: glucagon (cAMP → PKA → phosphorylase kinase) activates it; free glucose and insulin inactivate it.
- Muscle: adrenaline activates it, as do Ca²⁺ from contraction (through phosphorylase kinase) and AMP directly.
- ATP and glucose 6-phosphate inhibit it in muscle.

## Inhibitors and toxins

- Glucose (liver)
- Caffeine (nucleoside site, weak effect)
- Synthetic inhibitors studied for type 2 diabetes

## Clinical notes

- Muscle deficiency (PYGM): McArdle disease, with cramps, myoglobinuria and the second-wind phenomenon.
- Liver deficiency (PYGL): Hers disease, with hepatomegaly and mild hypoglycaemia.
- Phosphorylase kinase deficiency (GSD IX) gives a liver picture similar to Hers disease.

## Pathways

- [Glycogen metabolism](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/)

## Related conditions

- [McArdle disease (glycogen storage disease type V)](https://durgical.com/en/conditions/mcardle-disease/)
- [Hers disease (glycogen storage disease type VI)](https://durgical.com/en/conditions/hers-disease/)

## Related

- [Glycogen](https://durgical.com/en/biochemistry/molecules/glycogen/)
- [Glycogen synthase](https://durgical.com/en/biochemistry/enzymes/glycogen-synthase/)
- [Glycogen debranching enzyme](https://durgical.com/en/biochemistry/enzymes/glycogen-debranching-enzyme/)
- [Glucose-6-phosphatase](https://durgical.com/en/biochemistry/enzymes/glucose-6-phosphatase/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which vitamin cofactor does glycogen phosphorylase carry?

Pyridoxal phosphate, derived from vitamin B6.

## Why is it better to cleave glycogen with phosphate rather than water?

Glucose 1-phosphate comes out already phosphorylated: the hexokinase ATP is saved and the glucose stays trapped in the cell.

## What activates muscle phosphorylase during exercise?

Ca²⁺ from contraction (through phosphorylase kinase), AMP and adrenaline.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [PYGL: Glycogen phosphorylase, liver form (human)](https://www.uniprot.org/uniprotkb/P06737/entry) (UniProt, 2026); [PYGM: Glycogen phosphorylase, muscle form (human)](https://www.uniprot.org/uniprotkb/P11217/entry) (UniProt, 2026); [PYGM gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/pygm/) (MedlinePlus, US National Library of Medicine, 2026); [1FA9: HUMAN LIVER GLYCOGEN PHOSPHORYLASE A COMPLEXED WITH AMP](https://www.rcsb.org/structure/1FA9) (RCSB Protein Data Bank, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/glycogen-phosphorylase/